1. Field of the Invention
This invention relates to glow discharge heating devices in which energy is transferred to a workpiece by a fine-focus discharge.
2. Description of the Prior Art
For non-contacting energy transfer to effect heating of a workpiece, such as a fine wire or strip, a glow discharge device may be used. Such techniques are employed for example in the continuous treatment of wire or strip which is moved through the heating device. The power has to be concentrated in the spatial region through which the workpiece passes and this requires a fine focus of the electron beam with the focus location remaining stable over long periods of time.
The power delivered to the workpiece must always be less than that applied to the heating device and thus the electron gun assembly of a glow discharge heating device is subjected to power dissipation with consequent temperature rise. The increase in temperature, if not controlled, leads to dimensional changes and variability in performance until a steady state condition is achieved. Typically, with existing types of glow discharge heating devices for heating wire or strip, a period of 20-25 minutes might be required to achieve a steady state. During this time the workpiece has not experienced the correct heat treatment. This long delay period and consequent waste of material may make the whole process uneconomic.
Although, in principle, this problem could be overcome by the use of materials for the heating device having little or no temperature coefficient of expansion, such materials, in practice, may not be suitable nor has it been found practicable to put a coating of a suitable material on a dimensionally stable structure.